You're right, there's definitely a bug or something strange with that
TLDR : Do not use any "​ inside your sender address in Odoo

if you set (inside Emails Marketing > Settings) the sender address to be
> "Monsieur Biche - Hello !" <hello@monsieur-biche.fr>​
=> Here's what you will see inside the mail source (=> you will get the incorrect display name)
> From: "Monsieur,Biche,-,Hello,!" <hello@monsieur-biche.fr>​

but if you use
> Monsieur Biche - Hello ! <hello@monsieur-biche.fr>
Here's what you will see inside the mail source (=> you will get the correct display name)
> From: "Monsieur Biche - Hello !" <hello@monsieur-biche.fr>​

Thank you for your hints, hope it will help others ! 😉